The invention relates generally to a fuel cell stack and more particularly to a sealing process of the fuel cell stack.
Fuel cells produce electricity by oxidizing fuel on one electrode (anode) and reducing oxygen on the other electrode (cathode). The electrodes are separated by an electrolyte that conducts electricity by the migration of ions. Under the appropriate conditions the reduction/oxidation reactions on the electrodes produce a voltage, which can then be used to generate a flow of direct current. In the case of a solid oxide fuel cell operating with hydrogen fuel and air as an oxidant, oxygen ions are conducted through the electrolyte where they combine with hydrogen to form water as an exhaust product. The electrolyte is otherwise impermeable to both fuel and oxidant and merely conducts oxygen ions. This series of electrochemical reactions is the sole means of generating electric power within the fuel cell. It is therefore desirable to reduce or eliminate any mixing of the reactants that results in a different combination such as combustion which does not produce electric power and therefore reduces the efficiency of the fuel cell.
The fuel cells are typically assembled in electrical series in a fuel cell stack to produce power at useful voltages. To create a fuel cell stack, an interconnecting member, referred to as an interconnect, is used to connect the adjacent fuel cells together in electrical series to form a fuel cell assembly. Typically, an anode layer is connected to an anode interconnect and a cathode layer is connected to a cathode interconnect. When the fuel cells are operated at high temperatures, such as between approximately 600° C. and 1000° C., the fuel cells are subjected to mechanical and thermal loads that may create strain and resulting stress in the fuel cell stack.
Typically, high temperature fuel cells are made of ceramics, which must be sealed to the metallic interconnect structure in order to define closed passages for reactants, namely the fuel and the oxidant to flow to and from the fuel cell. During the thermal cycles of the fuel cell assembly, various components of the fuel cell stack expand and/or contract in different ways due to the difference in the coefficient of thermal expansion of the materials of construction. In addition, individual components may undergo expansion or contraction due to other phenomena, such as a change in the chemical state of one or more components. This difference in dimensional expansion and/or contraction may affect the seal separating the oxidant and the fuel paths and also the sealing of the elements made of dissimilar materials.
Conventionally, a typical anode layer of a fuel cell is made of a nickel based cermet, which itself is made by chemical reduction of nickel oxide in mixture with a ceramic. A major problem in fuel cell stack design is that the high temperature typically requires that the seals be made of brittle materials such as glass and glass ceramics. Prior to operation, the nickel oxide in the anode of the fuel cell is reduced to nickel at high temperature, and this chemical reduction causes a physical reduction of volume of the anode. This reduction in the volume of the anode layer can place additional stress on links between the fuel cell and other components, such as the seal, and can cause the seal of the fuel cell assembly or the fuel cell itself to fail. This is aggravated by the stresses arising from different coefficients of thermal expansion of the ceramic and metal, thereby causing the unequal physical reduction of volume of the anode layer and the interconnect in contact with the anode layer. Another consequence of the differential thermal and chemical expansions of the fuel cell and the interconnect is the potential loss of mechanical contact between the anode layer or cathode layer and its corresponding interconnect (the anode interconnect or the cathode interconnect).
In addition, conventional processing of multiple fuel cells in a fuel cell stack has relied upon sealing all or several of the fuel cells and interconnects in a single process to form an integral, inseparable stack. If, following such assembly and processing, a defect is identified in any seal of the fuel cell stack, the fuel cell stack cannot be disassembled without destroying the seals. This means that any defect in the fuel cell stack could render the entire fuel cell stack unusable.
A common approach to the thermal stress problem is to find a combination of ceramic and metal where the coefficients of thermal expansion match closely enough that stresses are minimized. However, it is very difficult to match the coefficients over the entire temperature range. Moreover, even such matching does not avoid stresses due to the reduction in volume of the anode layer in its pre-operation transition from a ceramic and nickel oxide mixture to a nickel based cermet. Also, the materials chosen based upon a close thermal match may not be optimal for the performance of the fuel cell.
Therefore, there is a need to design a fuel cell stack that is compliant to changes in operating states including temperature cycles and changes in chemical state, and that permits the seal of the individual fuel cells in a fuel cell stack to be inspected before the final assembly.